1. Field of the Invention
The present invention relates in general to a method of fabricating nanostructured materials. More particularly, it relates to a method of templating synthesis of nanostructured materials in functionalized mesoporous materials.
2. Description of the Related Art
Nanostructured materials have unique optical, electrical, magnetic and mechanical properties compared with bulk materials. The reduction of the size of the material results in not only the quantum confinement phenomena due to the changes in the density and gap of the electronic energy level, but also the increase of the ratio of surface atoms to bulk atoms. Therefore, they have found extensive applications in catalysis as well as miniaturized electronic and optical devices. Nanostructured materials include nanoparticles, nanorods and nanowires. Template synthesis within a confined space of host, or xe2x80x9cship-in-a-bottlexe2x80x9d synthesis, is an effective method to prepare uniform sized nanomaterials. The morphology of nanomaterials can be controlled by the void structure of the host. Examples of suitable hosts include anodic alumina membranes (Ex: U.S. Pat. No. 6,231,744 B1), carbon nanotube (e.g. J. Sloan et. al., J. Chem. Soc., Chem. Commun., (1999) 699; A. Govindaraj et. al., Chemistry of Materials, 12 (2000) 202), self-assembled diblock copolymer template (T. Thurn-Albrecht et. al., Science 290 (2000) 2126) and molecular sieves (Ex: M. Sasaki et. al., Microporous and Mesoporous Materials 21 (1998) 597; Y. J. Han et. al., Chemistry of Materials 12 (2000) 2068; Z. Liu et. al., Angew. Chem. Int. Ed. 39 (2000) 3107). In addition, various metal nanoparticles were synthesized in the confined space of micelles consisting of surfactant or polyelectrolytes (Ex: U.S. Pat. Nos. 5,925,463, 6,054,507, 6,090,746, 6,099,964, 6,103,868, 6,262,129 B1 and 6,325,910 B1). Another method of fabricating nanoporous materials involves direct templating of lyotropic liquid-crystalline phases of amphiphilic surfactants or block-copolymer (U.S. Pat. Nos. 6,054,111 and 6,203,925 B1). Most of these hosts provide spherical voids or one-dimensional channels to fabricate nanowire structures. In comparison, surfactant templated, ordered mesoporous molecular sieves including MCM-series and SEA-series mesoporous silicas, have tunable pore size in the range of 1.0-50.0 nm, as well as controllable three-dimensional pore structures (U.S. Pat. No. 5,145,816 and 5,264,203; Q. Huo et.al., Nature 368 (1994) 317; D. Zhao et. al., Science 279 (1998) 548). They can be fabricated into various morphologies including powder, fiber, monolith and thin film, which provide further flexibility in processing nanomaterials. Therefore, mesoporous materials are promising hosts for fabrication of nanostructured materials.
Template synthesis of nanostructured materials, such as metals, metal oxides or semiconductors, in mesoporous materials generally starts from incorporation of suitable precursors. Several methods including incipient wetness impregnation, ion exchange, or gas-phase chemical vapor infiltration are utilized. For practical usage of mesoporous materials as host, either high loading or high degree of dispersion of nanomaterials in the host is critical. For these incorporation methods, repeated incorporation cycles or long vapor treatment time is usually needed to achieve high precursor loading. The dispersion degree is usually low, and the precursors diffuse easily to the outer surface of the host to form large materials during subsequent processes. Therefore, an efficient method for fabricating nanostructured materials in mesoporous hosts is needed.
In a separate field of environmental chemistry, methods have been developed to functionalize mesoporous silica to serve as sorbents for separation and sensors. The pore surface of mesoporous silica is functionalized to carry sulfur or nitrogen-containing functional group, which can react and adsorb metal ions in the solution (U.S. Pat. Nos. 6,251,280 B1, 6,306,301 B1, 6,310,110 B1 and 6,326,326 B1). The coverage of surface functionality can be finely tuned so that a monolayer of functional groups on the pore surface of mesoporous silica can be formed. The interaction between functional groups and metal ions is relatively weak, and limited amount of metal ions can be adsorbed on the pore surface of mesoporous silica. There remains a need to explore the possibility of applying surface functionalization in fabricating nanostructured materials in mesoporous hosts.
The object of the present invention is to provide a method of fabricating nanostructured materials in functionalized mesoporous materials. Mesoporous materials are functionalized to bear charged functional groups on the pore surface, and are applied for template synthesis of nanostructured materials. Long-ranged electrostatic interaction between the surface charged groups and the precursor salts results in high precursor loading in the pore of the host. In addition, the precursor distributes uniformly in the functionalized mesoporous host. After subsequent reaction, such as reduction, oxidization or complexation, the resulting nanostructured materials will have high degree of dispersion.
Mesoporous host material is preferably to maximize the number density of active groups on the pore surface by, for example, rehydration process. It then reacts with functional molecules to form a monolayer of positively or negatively-charged functional groups on the pore surface of mesoporous host. A solution of metal salts or molecules bearing opposite charges is mixed with the functionalized mesoporous host, and may be ion exchanged into the mesopores. Alternatively, metal salts or molecules with high concentration in the solution may nucleate and precipitate inside the mesopores, resulting high loading of the complex. Finally, the confined metal salts or molecules in the mesopores can be reduced or oxidized to form nanostructured metal or metal oxide; or it may react with secondary ligands or reactants to form functional nanostructured crystals or precipitates inside the mesoporous host materials.
According to the present invention, the nanostructured materials are not restricted to metal and metal oxide. Insoluble metal salts or metal-ligand hybrids can also be fabricated inside the mesoporous hosts. Appropriate charged molecule or metal complex is first incorporated into the functionalized host. Upon introduction of the ligand, the insoluble hybrid precipitates immediately inside the mesopores.
In the present invention, the template-synthesized nanostructured materials not only impart mechanical reinforcement to the host mesoporous material, but also extend the applications of the nanocomposites. The applications depend on the properties of the nanoparticles. For example, Fe2O3 can be used as a magnetic recorder, while Pt nanoparticles perform as better catalyst for hydrogenation reactions. Confined bimetallic RuPt or PdAg nanoparticles may be used as catalysts for fuel cell applications.